The enactment and enforcement of various environmental protection laws, especially those designed to improve water quality, have made it desirable to significantly reduce or eliminate the discharge of cyanides, phosphates, and a number of metal ions, from the effluents of electroplating plants. As a result, non-polluting bright zinc plating processes have been sought as alternatives to the classical zinc cyanide baths.
Alkaline solutions containing complex compounds of zinc and alkaline metal pyrophosphates have been proposed as a replacement for cyanide baths and cyanide processes for the electrodeposition of bright zinc. The electrodeposition of zinc using a pyrophosphate bath, however, may give relatively poor low current density coverage, spore formation, roughness, insufficient brightness, and relatively non-uniform deposits. In addition, passivation of the anodes may produce undesirable precipitates which in turn can clog filter systems and sometimes results in intermittent operation necessitated by frequent changes of filter media.
The use of phosphates may also produce waste disposal problems since phosphates are not easily removed and may promote the growth of undesirable aquatic plant if discharged into streams. These disposal disadvantages further limit the acceptance of pyrophosphate zinc plating bath compositions in industrial applications.
Non-cyanide zincate zinc plating baths have also been proposed as substitutes for cyanide containing systems. However, the bright plating current density range of these baths is quite limited, making the plating of articles of complex shape difficult, if not impossible. Since the addition of cyanide to these non-cyanide zincate baths greatly improves the bright plate current density range of the deposits, platers tend to add cyanides to their zincate systems, thus negating the non-cyanide feature of the original bath.
Highly acidic zinc plating baths have been known for some time and such baths are cyanide-free. These systems do not produce bright decorative deposits, (in the currently accepted usage of the word "bright"), have extremely poor low current density coverage and find their chief application in the strip line plating of wire and sheet steel using very high but narrow current density ranges. Thus, they are not suited for plating objects of complex shape or for normal decorative, or rustproofing application.
Neutral, mildly alkaline or mildly acidic non-cyanide zinc plating baths containing large amounts of buffering and complexing agents to stabilize pH and solubilize the zinc ions at the pH values involved have been employed to overcome the objections of using cyanide-based zinc plating processes.
In order to improve and increase the brightness, luster and throwing power of zinc deposits from these baths, certain organic aromatic carbonyl compounds are generally used as brighteners.
These brighteners provide fairly satisfactory zinc deposits, but the deposits tend to be dull in the low current density regions, and they have a limited solubility in mildly acidic zinc electrolytes.